I just finished Simon LeVay’s Gay, Straight, and the Reason Why, which summarizes what science knows about sexual orientation.  LeVay himself contributed to the science, discovering in 1991 that a tiny region of the hypothalamus called INAH3, normally larger in men than women, is smaller in gay men (indeed, it’s the size it is in women). 

He summarizes a bunch of studies which have found all sorts of differences between men and women, and then between straights and gays/lesbians.  Quite a few of the effects are small or disputed– I’d really, really like to get a version of this book from about 2050 when we will know so much more.  But it’s fair to say that the evidence points to a biological basis for homosexuality, and against any social, developmental, or moral basis.  (E.g., there’s evidence for the inheritability of homosexuality, and this holds up when e.g. siblings or twins are raised apart, so it’s unlikely to be the result of the environment.  There’s also good evidence that “pre-gays/pre-lesbians” can be identified in childhood, when the environment really hasn’t had much time to work.  And you can mess with animal genes and produce mating behavior aimed at the same sex.)

(Also, contra Bagemihl, there isn’t so much evidence for exclusive homosexuality among animals.  E.g. the famous gay geese often mate with female geese when they’re available.  For some reason there are a lot of gay rams though.)

The most interesting bit is a new answer to the perennial question of why exclusive homosexuality persists when it presumably reduces one’s chances of raising offspring.  (I say presumably because I’m not convinced that we know it does so in the ancestral environment.  But it’s still something we’d like a good explanation for.)  There are old ideas such as that homosexuals spend more time helping their nieces and nephews, but that’s not very convincing and hasn’t been supported.

But Edward Miller has another idea: there may be a number of genes which increase feminization– e.g. empathy, kindness, reduced aggressiveness.  Get one or a few of these traits and they make a man more attractive to women— i.e. they increase reproductive success.  Get them all, and you end up gay.  Psychology Today calls it the “Johnny Depp effect”.

Even more interesting, predictions made by this model have been tested and seem to bear up: a test of 5000 Australian twins found a) that increased ‘femininity’ among straights led to an increase in female sex partners, and b) heterosexuals with gay twins had more opposite-sex partners.  Similar results held with lesbians and straight women.

The most surprising finding from LeVay’s book, at least to me, is that gayness is associated with a whole slew of feminized structures or behaviors, and lesbianism with masculinized ones.  It really sounds like disreputable old attitudes (gay men are girly, lesbians are mannish) that I’ve taken as simplistic, misleading, and even offensive.  E.g. there’s a point in Kiss of the Spider Woman where the gay man declares that he identifies with women “always”, and that struck me as completely wrong… the gays I know identify with gays and don’t strike me as being particularly feminine.  Calling gays feminine seems as wrong as describing Englishmen, to Americans, as “partly French”.  Maybe they’re, I dunno, a bit Frenchier than Americans, but isn’t it better to recognize that Englishness is a separate thing from the American/French continuum?

To be sure, most of the studies LeVay talks about do deal with continuums; there’s a tendency to shift the behavior in the direction of the other sex, but it’s a) often incomplete and b) often pretty scattershot. 

There’s one huge gap, which LeVay is quite aware of: there is not much discussion of bisexuality, or different types of gays/lesbians, or how all this goes down in very different cultures, or why some people switch over late in life, or gender dysphoria that’s not linked to sexual orientation.  This isn’t much addressed simply because it hasn’t been studied as much… it’s hard enough to get a large sampling of gays and lesbians, much less get them all sorted out into butch/femme or whatever.  But some of these factors strike me as very important.  In Latin American culture (as he mentions, in fact), those who identify as homosexual are usually bottoms, while tops may perceive themselves as entirely hetero.  That’s awfully hard to fit into a framework that considers gay and straight to be fairly separate categories. 

LeVay leans toward explanations that rely on an increased or decreased supply of testosterone in the womb… something that can be affected by genes as well as some random environmental factors.  Which is probably as close as we’re going to get right now; I wonder how close it’ll look to whatever they’re positing in 2050.

This isn’t terribly light reading– LeVay goes over a lot of details about neurons and genes and finger ratios and sampling techniques, and it’s best if you’re comfortable with this sort of academic tone.  But it’s interesting to see what we know so far about the subject.